1. Field of the Invention
The present invention relates to a particulate filter for trapping particulates contained in exhaust gas.
2. Description of the Related Art
In a vehicle equipped, for example, with a lean-burn gasoline engine or a diesel engine, carbon-containing particulates (hereinafter referred to as “particulates” or “particulate matter (PM)”) are contained in exhaust gas discharged from the engine. Therefore, with a view to suppressing emissions of PM, a particulate filter is provided in an exhaust passage to trap PM thereon.
In such a vehicle provided with the particulate filter, an increase in PM trap amount (i.e., amount of PM trapped on the particulate filter) is likely to cause problems such as a reduction in engine power and an increase in fuel consumption. Therefore, when the PM trap amount is increased up to a given value, a particulate-filter regeneration process is performed to burn and remove the trapped PM from the particulate filter.
As one example of the particulate-filter regeneration process, there has been known a technique of providing an oxidation catalyst on an upstream side of the particulate filter in an exhaust-gas flow direction, and inducing a reaction between HC and CO components in exhaust gas and the oxidation catalyst so as to increase a temperature of the particulate filter by heat of combustion of the HC and CO components to burn PM trapped on the particulate filter.
A honeycomb-structured particulate filter is commonly known in which a plurality of inflow cells each extending in an exhaust-gas flow direction and having an open end on an upstream side in the exhaust-gas flow direction and a closed end on a downstream side in the exhaust-gas flow direction, and a plurality of outflow cells each extending in the exhaust-gas flow direction and having an open end on the downstream side in the exhaust-gas flow direction and a closed end on the upstream side in the exhaust-gas flow direction, are defined by a cell wall having fine communication pores for allowing exhaust gas to pass therethrough, in such a manner that the inflow cells and the outflow cells are arranged in a checkered pattern.
In the honeycomb-structured particulate filter, as well as a widely-used type designed to form each of the inflow cells for inflow of exhaust gas to have an opening area approximately equal to that of each of the outflow cells for outflow of exhaust gas, there has been known another type designed to form each of the inflow cells to have an opening area greater than that of each of the outflow cells (see, for example, Japanese Utility Model Laid-Open Publication No. 58-092409 and Japanese Patent Laid-Open Publication No. 2005-125209).
As compared with the first type of particulate filter designed to form each of the inflow cells to have an opening area approximately equal to that of each of the outflow cells, the second type of particulate filter designed to form each of the inflow cells to have an opening area greater than that of each of the outflow cells, as disclosed in the Japanese Utility Model Laid-Open Publication No. 58-092409 and the Japanese Patent Laid-Open Publication No. 2005-125209, can reduce a thickness of accumulated PM to suppress a pressure loss therein, under a condition that the same amount of PM is trapped by the two types of the particulate filters. This makes it possible to increase a time interval between the particulate-filter regeneration processes, and suppress the reduction in engine power and the increase in fuel consumption.
In addition to PM, ash comprising a glassy compound containing metal and phosphorus in engine oil is contained in exhaust gas discharged from the engine. While this ash is also trapped by a particulate filter provided in the exhaust passage, it will be continuously accumulated on the particulate filter without being burnt and removed even after being subjected to the particulate-filter regeneration process. Thus, the second type of particulate filter designed to form each of the inflow cells to have an opening area greater than that of each of the outflow cells is effective for reducing a thickness of accumulated ash to suppress a pressure loss therein.